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Comets are the remnants of the formation of the solar system 4.6 billion years ago. Scientists hope Rosetta will unlock the secrets of their chemical make-up and reveal whether they once brought water, and even the building blocks of life, to Earth.

Dr Gerhard Schwehm, Rosetta's main scientist, said: "We will look back to the infant stage of the solar system when planets were formed out of a cloud of dust and gas."

The probe was partly built by EADS Astrium in Stevenage. Ten of its 21 instruments involved British scientists, while the UK has contributed £70 million to the £600 million unmanned mission.

It was originally scheduled to launch last year to study the comet Wirtanen but was postponed after the failure of an Ariane 5 rocket in 2002. The three-ton craft is now scheduled to launch from Kourou, French Guiana, on Thursday.

Rosetta, named after the Rosetta Stone, a slab of basalt which was the key to unlocking Egyptian hieroglyphics, is an aluminium box, 9ft by 6ft by 6ft. Its solar arrays are huge, stretching to nearly 100 feet.

Rosetta needs just 400 watts of power, the equivalent of four light bulbs, and will be the first spacecraft to rely on solar power so far out from the Sun.

It will have to cope with temperatures as cold as -150C and as warm as 30C. During its rendezvous, radio signals will take 50 minutes to reach Earth.

No rocket is powerful enough to take Rosetta all the way to the comet so it must bounce around the solar system, using the gravitational force of planets to pick up speed.

It will orbit the Sun four times, enter the asteroid belt twice, swing past Mars in 2007 and fly by the Earth in 2005, 2007 and 2009. It will reach the comet's orbit in May 2014. Over the next six months it will edge closer to its destination until it has matched its speed.

It will start to map the comet's surface when it is about 15 miles away. Once it has found a suitable landing site, it will release the lander - named Philae after an island in the Nile where an obelisk was found that gave Egyptologists the final clues needed to decipher the Rosetta Stone.

The 220lb lander will smack into the comet at walking speed and immediately release two harpoons to attach it to the surface. Once in position it will drill into the icy crust, analyse the terrain's make-up and take the most detailed images ever of a comet.

The lander and orbiter will continue collecting data about the make-up of the comet, sending it back to Earth as the comet approaches the sun and begins to melt, releasing a stream of dust and gas. The mission should end in December 2015.

Dr Ian Wright, of the Open University, who is the main scientist behind the lander's Ptolemy gas analyser, said: "It's entirely possible that the water on Earth was brought by comets." No one knows what the surface of the comet nucleus will be like but "the lander has been designed to cope with concrete or snow", he added.

Lord Sainsbury, the science minister, said: "The Rosetta mission could provide answers to the questions of how life actually began."